A wide variety of techniques for depositing and etching layers of material at the surface of a substrate are known. The deposition techniques include liquid and vapor phase chemical deposition, epitaxial crystal growth, ion sputtering and molecular beam epitaxial growth, to name a few. The etching techniques include wet etching, plasma etching, ion-assisted etching, laser drilling and physical sputtering, to name a few. Each of these techniques possess advantages and disadvantages that may vary depending on the circumstances in which the technique is used. The advantages and disadvantages include low deposition and removal rates, non-uniformity over large surface areas, isotropic/anisotropic effects, induced crystal lattice damage and substrate surface contamination.
One criteria often encountered in selecting any particular deposition or removal technique is commercial viability. Typically, the technique must act effectively with commercially acceptable substrate diameters, processing rates and materials. Currently, most commercial material substrate diameters exceed one inch.
Another criteria is the resultant surface morphology and associated defect density. Since protuberances, contaminants and crystal lattice defects may act as undesirable points of heightened or non-uniform processing activity, the deposition or etching technique should provide an acceptably smooth, clean and undamaged surface.
Conventional effusive source molecular beam techniques represent a technology that, in general, meets many of the above criteria. Conventional molecular beams are created by the effusive release of a reactant species into a highly evacuated chamber at a point directly opposing a typically flat substrate surface. Due to very low beam gas pressure, the individual particles of the effusively released reactant species largely maintain their respective initial thermal velocity and divergent directions of travel. The effusive release of the reactant species under such circumstances has been well characterized as having a cosine-angular distribution. The typically flat substrate surface therefore receives the molecular beam with a non-uniform flux over the beam-incident portion of its surface. The non-uniformity in incident flux produces a corresponding non-uniform deposition or etching of the substrate surface, though such non-uniformities can be reduced by mechanically displacing the substrate with respect to the beam. However, a significant detracting consequence of the effusive nature of molecular beams is that the total flux of the beam is quite low, particularly as compared to other techniques that utilize gas flows at much greater relative pressures.
One such other technology, utilizing a nozzle beam generated from a high pressure source, realizes a very high total-beam flux. A nozzle beam is formed when the source gas is injected into a vacuum chamber, under extremely high pressure to effectively convert the random thermal motion of the source gas molecules to directed translational motion. The resultant nozzle beam is monoenergetic and has a directionally peaked angular distribution. The major drawback limiting the effective use of nozzle beam technologies is the very large primary vacuum pumping system required to handle the substantial gas load that must be continuously evacuated from the chamber. Further, the sharp angular distribution results in a more non-uniform beam intensity on the substrate as compared to a conventional effusive beam.
In an effort to improve on molecular beam technology, some research has been done to characterize a multichannel molecular beam source. The effort undertaken was to characterize the angular and velocity distributions of an effusive gas emitted from a glass tube bundle used as the beam source. The source bundles contained less than about 4000 channels with each channel being 0.025 cm long and 11 micrometers in diameter. The micro-channels where packed into a bundle of only about 1 millimeter diameter or less.
These multichannel source characterization efforts were primarily for the fundamental beam surface scattering studies of gas-solid interactions. In these experiments, the primary molecular beam interacts with another beam or a surface before detection. The interaction zones in these studies have a desirably, well-defined cross-sectional area of about 1 millimeter.sup.2 or less. These studies require accurate knowledge of the intensity, angular and velocity distributions of the gas molecules injected into the interaction zone.
In characterizing the multichannel sources, the most well-defined zone was obtained with a molecular beam utilizing thin-wall orifices operated at source pressures low enough to ensure free molecular flow conditions. The multichannel sources where studied to relate the size and general structure of the microchannel source to the total flow and angular/velocity distribution beam that they produce. As a result, those multichannel sources studied were generally characterized with regard to their predicted angular/velocity distribution function. However, the experimental results, for pressures higher than that for free molecular flow associated with the diameter of the channels tested, were not accurately in agreement with the available theoretical predictions. Therefore, multichannel sources have generally not been considered suitable for fundamental studies as compared to single channel source effusive beams.
A purpose of the present invention is, therefore, to realize a large cross-sectional area, low-divergence, directionalized molecular beam for use in either depositing or etching material at the surface of a substrate.